1. Field of the Invention
This invention relates to the method of recovering T.U.D. dissolved in the waste liquor which is produced in the production of thiourea dioxide (hereinafter referred to as "T.U.D.").
2. Description of the Prior Art
Among the methods of production of T.U.D. there have been announced various reports on the method of its production by reaction of thiourea and hydrogen peroxide in a solvent.
The solvents used in such reported methods are broadly classified into non-aqueous and aqueous solvents.
The production of T.U.D. by the reaction of thiourea and hydrogen peroxide in a non-aqueous solvent (chlorinated solvents such as crbon tetrachloride and chloroform as well as lower aliphatic alcohols) has been proposed by German Pat. No. 917553, Italian Pat. No. 579119 and French Pat. No. 2040797. This proposed method is advantageous in that T.U.D. as the reaction product can all be recovered as cryatals because it does not dissolve in the solvent. But such method is disadvantageous in that since the reaction of thiourea and hydrogen peroxide is a heterogeneous reaction, there are produced by-products in large quantities and these by-products do not dissolve in the solvent, so they are incorporated into the product T.U.D., thus causing the purity of the product to deteriorate, and besides the solvent used is lost in large quantities. Thus, an industrial adoption of such method involves many disadvantages.
On the other hand, in the case of production of T.U.D. in an aqueous solvent, the starting thiourea dissolves well in water and forms homogeneous phase. Since the reaction of thiourea and hydrogen peroxide is very fast, their reaction in a homogeneous phase is advantageous in that the formation of by-products is less in quantity and T.U.D. can be obtained in high yield and high purity.
However, the said method is disadvantageous in that the quantity of T.U.D. crystals obtained becomes smaller because part of the reaction product T.U.D. is dissolved in the waste liquor after separation of the T.U.D. crystals formed.
To eliminate this drawback, U.S. Pat. No. 2,783,272 and Japanese patent publication No. 17665/1970 propose a method in which such waste liquor is re-used as a reaction solvent to recover T.U.D. dissolved therein. According to such method, however, a repeated re-use of the waste liquor causes a correspondingly increasing accumulation in the reaction solution by-products, e.g. sulfuric acid, as well as impurities contained in the starting thiourea and hydrogen peroxide, resulting in that the yield and purity of T.U.D. lower.
Japanese patent publication No. 17765/1970 proposes a method in which the waste liquor is treated with a carbonate or hydroxide of an alkaline earth metal, which is a metal of Group IIa of the Periodic Table, to neutralize the sulfuric acid contained therein, and after separation of the resulting precipitate of a sulfate the waste liquor is re-used as a reaction solvent.
Treating the waste liquor in such a manner somewhat improves the yield and purity of T.U.D. as compared with the case where the said treatment is not applied, but does not afford a satisfactory result.
Although the above treating method can remove sulfuric acid contained in the waste liquor, it cannot remove other side reaction products and impurities derived from the starting thiourea and hydrogen peroxide, so that also in this treating method impurities accumulate in the reaction solution as the re-use of the treated waste liquor is repeated, which impurities not only hinder the reaction of thiourea and hydrogen peroxide but also contaminate the reaction product T.U.D., and thus a satisfactory result is not obtained. According to the Japanese patent publication referred to above which proposes this treating method, the neutralizer used therein does not promote the decomposition of T.U.D. However, in case such neutralizer is added in excess by mistake, there occurs a vigorous decomposition of T.U.D.; therefore, even if the waste liquor after neutralization is re-used to prepare T.U.D., there can be expected no improvement in the yield of T.U.D.
Furthermore, the neutralizer used in the waste liquor treatment is dissolved in the treated waste liquor, so the re-use of the latter causes lowering of the purity of the resulting T.U.D. This drawback is pointed out in Japanese patent public disclosure No. 62934/1975.
From a common-sense point of view, the evaporation method is considered as the method of recovering T.U.D. contained in the waste liquor, but an industrial adoption thereof has been impossible because of the following drawbacks: at the time of concentration T.U.D. undergoes a thermal decomposition resulting in decrease of its quantity recovered, and since the by-products dissolved in the waste liquor as well as the impurities derived from the starting thiourea and hydrogen peroxide are recovered at the same time, the purity of the recovered T.U.D. lowers.